The present invention relates in general to calibration and alignment of a metrology device and, in particular, to aligning the transmission axis of a polarizer with a sample, such as a diffraction grating.
It is desirable to measure circuit structures and other types of structures, e.g., resist structures, during the production of integrated circuits. Optical metrology tools are particularly well suited for measuring microelectronic structures because they are nondestructive, accurate, repeatable, fast, and inexpensive. Often different metrology tools are required to measure different structures or parameters on a wafer. For example, certain structures on a wafer act as diffraction gratings, which conventionally require a different metrology tool, e.g. critical dimension-scanning electron microscopy (CD-SEM), than is used to measure planar thin films.
One particularly useful instrument to characterize the critical dimensions (CDs) of a diffraction grating, or other microcircuit structures, is a normal incidence polarized reflectance spectrometer, such as that described in the U.S. Patent Application entitled xe2x80x9cApparatus and Method for the Measurement of Diffracting Structures,xe2x80x9d by J. Holden, W. McGahan, R. Yarussi, P. Rovira, and R. Lowe-Web, filed Sep. 25, 2000, having Ser. No. 09/670,000, having the same assignee as the present disclosure, and which is incorporated herein by reference. Among many advantages of this tool are the relatively easy calibration and alignment procedures compared to other types of metrology devices, as well as the adaptation to a polar coordinate, i.e., R-"THgr", stage, which is particularly suitable for integrated metrology. Because the measurements are at normal incidence, the polarizer can be aligned either perpendicular or parallel to the lines of the grating structure, i.e., the TM or TE axes, which speeds up the modeling of the reflectance spectrum because only one component (either RTM or RTE) needs to be calculated.
To achieve accurate results with the normal incidence polarized reflectance spectrometer, a calibration and alignment procedure is used to precisely align the polarizer transmission axis with the lines of the diffraction structure. A methodology for this alignment process is based on a Jones vector formalism to obtain the reflectance as a function of the polarizer angle with respect to the lines of the grating structure. Defining the plane parallel to the lines of the grating, i.e., TE, as a reference, the following equation is obtained:                               R          ⁡                      (            P            )                          =                                            R              TE                        ·                                          cos                4                            ⁡                              (                                  P                  -                                      P                    S                                                  )                                              +                                    R              TM                        ·                                          sin                4                            ⁡                              (                                  P                  -                                      P                    S                                                  )                                              +                      2            ·                                                            R                  TE                                ·                                  R                  TM                                                      ·                          cos              ⁡                              (                Δ                )                                      ·                                          sin                2                            ⁡                              (                                  P                  -                                      P                    S                                                  )                                      ·                                                            cos                  2                                ⁡                                  (                                      P                    -                                          P                      S                                                        )                                            .                                                          eq        .                  xe2x80x83                ⁢        1            
In equation 1, P is the angle between the polarizer transmission axis and the polarizer""s home position, PS is the polarizer offset angle between the polarizer transmission axis and the lines of the diffraction grating, and xcex94 is the phase difference between the reflected fields in the TE and TM directions, i.e., parallel to and perpendicular to the lines of the diffraction grating.
Using equation 1, one of the proposed methods used to precisely align the polarizer transmission axis with the lines of the diffraction structure in Ser. No. 09/670,000 was to measure several R(P) spectra collected at different polarizer angles, e.g., ranging from 0 to 180 degrees, and obtain RTE, RTM, cos(xcex1) and PS in a fitting routine or a Fourier transform approach. A simulation of R(P) for a given wavelength as a function of the angle between the polarizer transmission axis and the polarizer""s home position is shown in FIG. 8, by way of example. Consequently, the polarizer offset angle PS can be determined and the polarizer aligned with the lines of the diffraction grating by rotating the polarizer to either PS or PSxc2x190 degrees to obtain RTE or RTM spectra, respectively. The alignment process, i.e., measuring R(P), fitting the measured R(P) to equation 1, determining the polarizer offset angle PS, and rotating the polarizer by PS or PSxc2x190 degrees must be performed for each substrate that is loaded onto the spectrometer. While one of the main advantages of normal incidence polarized reflectance is speed with which the modeled data can be calculated (because only one of either RTE or RTm spectra need be collected), the total measurement speed of the system is reduced because of the time consuming alignment process required for each new substrate.
In addition, other metrology devices, such as ellipsometers with rotatable polarizers, may be used to measure diffraction gratings. It is desirable to align or know the angle between the polarizer transmission axis of an ellipsometer with the lines of the diffraction grating to be measured.
Therefore, an efficient calibration and alignment procedure to determine and compensate for the polarizer offset angle PS is desirable.
A metrology device is calibrated to compensate for the polarizer offset angle PS, in accordance with the present invention, by first determining a system offset angle, defined as the angle between the transmission axis of the polarizer in its home position and an axis of motion of the stage that holds the substrate. The system offset angle is a constant for the metrology device, and therefore needs to be determined only once. For each substrate loaded into the metrology device, the sample offset angle is measured. The sample offset angle is defined as the angle between the axis of motion of the stage and the axis of orientation of the sample. The sample, may be, e.g., a diffraction grating, and the axis of orientation can be the TE or TM axes. The polarizer offset angle PS is equal to the system offset angle and the sample offset angle. Thus, to align the polarizer with the sample, the polarizer offset angle PS is reduced to zero by rotating the polarizer by an amount equivalent to the sum of the system offset angle and the sample offset angle. If desired, the polarizer may be rotated at separate times to compensate for the system offset angle and the sample offset angle. For example, the polarizer can be rotated to compensate for the system offset angle during an initial calibration and, thus, the polarizer""s home position to be aligned with the axis of motion of the stage. The polarizer can then be rotated-to compensate for the sample offset angle for each newly loaded substrate after the sample offset angle is measured for the newly loaded substrate. In another embodiment, the polarizer is rotated to compensate for the sum of the system offset angle and the sample offset angle for each newly loaded substrate. The polarizer is aligned with other samples on the same substrate by rotating the polarizer by the same angle that the stage is rotated to position the new sample for measurement.